In modern society, the occurrence of hearing loss is quite common, with approximately 10% of the population suffering from some degree of hearing impairment. This can be attributed to a number of causes, such as prolonged exposure to loud sounds, the result of disease or illness, or congenital problems.
Hearing loss is generally of two types, namely conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. In such cases, the hearing loss may often be improved by the use of conventional hearing aids, which amplify the sound so that acoustic information reaches the cochlea and the hair cells. Such hearing aids utilise acoustic mechanical stimulation, whereby the sound is amplified according to a number of varying techniques, and delivered to the inner ear as mechanical energy. This may be through a column of air to the eardrum, or direct delivery to the ossicles of the middle ear.
Sensorineural hearing loss, however, is due to the absence or destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. Individuals suffering from this type of hearing loss are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. In such cases, cochlear implants have been developed to provide the sensation of hearing to such individuals. In cochlear implants, electrical stimulation is provided via stimulating electrodes positioned as close as possible to the nerve endings of the auditory nerve, essentially bypassing the role of the hair cells in a normally functioning cochlea. The application of a stimulation pattern to the nerve endings causes impulses to be sent to the brain via the auditory nerve, resulting in the brain perceiving the impulses as sound.
As has been alluded to above, the treatment of both of these types of hearing loss has been quite different, relying on two quite different principles to deliver sound signals to be perceived by the brain as sound. It has been found that it is relatively common in hearing impaired individuals to experience sensorineural hearing loss for sounds in the high frequency range, and yet still be able to discern sounds in the middle to low frequency range, through the use of a conventional hearing aid, or naturally. Traditionally, in the majority of such cases, the individual would only receive treatment to preserve and improve the hearing for the middle to low frequency sounds, most probably via a conventional hearing aid, and little would be done to attempt to restore the hearing loss for the high frequency sounds. Only if the individual lost the ability to perceive the middle to low frequency sounds would consideration then be given to restoring the hearing loss for the high frequency sounds, in this case a cochlear implant would be considered a possible solution.
The specification for U.S. Pat. No. 6,231,604 introduces the concept of combining the two treatments, namely acoustic mechanical stimulation and electrical stimulation, for individuals with some degree of intact residual hearing. In this patent, the preferred embodiment makes mention of acoustic mechanical stimulation being used for sounds representative of low to mid-range frequencies in the acoustic environment, with electrical stimulation being used for sounds representative of mid to high-range frequencies in the acoustic environment. Whilst this patent identifies the need to attempt to combine the two stimulation methods it fails to suggest how such a system can be achieved, and the mechanism for performing this task.
International patent publication WO 00/69513 describes a number of embodiments of an electrode array that may be used to deliver electrical stimulation to the associated regions of the cochlea in order to supplement hearing of high frequency sounds. In this publication, a relatively short and thin electrode array is described as being between 6-8 mm in length and which is inserted through a small slit in the round window membrane for stimulation of the basal end of the scala tympani duct of the cochlea. In order to maintain the hydrodynamic nature of the cochlea, the described electrode array is provided with flexible flaps at its proximal end to assist in sealing the round window membrane and also to maintain the array in a position that is remote from both walls of the cochlea.
Experimental tests have shown that electrode arrays inserted to a depth as described by the above international patent publication will produce unnatural and sharp or high-pitched percepts in a recipient. Trials of such an array to a depth of 8 mm into the cochlea have indicated that recipients are not able to fuse the electrical stimulus with the auditory stimulus received. Therefore, the electrode array as described by the above-referenced patent publication will be unable to provide benefit to the recipient because of restriction in depth of insertion which is mandatory to preserve residual hearing. Further, the electrode array of the above-referenced patent publication will most likely cause damage to the basal membrane due to rotation or twisting of the array about its longitudinal axis. As the array relies upon flexible flaps for stabilisation and not the fixation of the head of the device, it is highly likely, due to the rounded shoulder of the device, that the array will not be stable within the cochlea, potentially causing damage to the sensitive structures therein. In addition, placement of such a device through the round window membrane may interfere with the micromechanics of the travelling wave of the inner ear.
The present invention is an attempt to address the perceived problems of such prior art devices.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.